In the production of semiconductor devices such as integrated circuits, circuit patterns are formed on silicon wafers by optical or electron beam lithography. A photomask, comprising a patterned opaque film on a transparent substrate, serves as the circuit pattern template in the lithography process.
Current trends in the semiconductor industry are towards increased circuit pattern density on the silicon wafers. As the circuit pattern density is increased, the permissible defect size and density on the photomask necessarily decrease. This decrease translates into fewer and smaller permissible defects in the photomask blank, a substrate having an opaque film thereon, from which the photomask is formed.
A primary source of defects in photomask blanks is the blank manufacturing process. Conventional photomask blanks include two or more different masking layers on the transparent substrate. A light blocking chrome or chrome-based layer and a chrome oxide antireflective layer are the basic masking layers. Additional layers such as further antireflective layers, etch rate enhancing layers and adhesion promoting layers are also used.
Typically, each masking layer is coated individually in separate coating operations. This is done, for example, by sputtering a chrome layer on an uncoated substrate in a sputter chamber, then removing the chrome coated substrate from the chamber, altering the conditions in the chamber to create a chrome oxide sputtering atmosphere and then subjecting the chrome coated substrate to the new conditions. This type of process has some disadvantages. Between coating the different layers, the coating surface is susceptible to contamination. The contamination may be in the form of solid particulates created by mechanical removal and return of the substrate from and to the chamber, or solid residue, particles or dust remaining in the chamber from the previous sputtering conditions. Moreover, the contamination may also be gaseous should there by any backstreaming of the vacuum pumping system between passes through the sputtering chamber.
Both forms of contamination reduce the adhesion at the coating interfaces in the final blank. Any adhesion loss, whether local or uniform, at any interface in the blank is a potential defect site in the final photomask. The rigorous processing steps of exposure, development, etching, stripping and numerous cleaning cycles, to which a blank is subjected in the manufacture of a photomask, enhance the likelihood that a given adhesion loss in a blank will generate a defect site in the photomask produced therefrom.
Another disadvantage of coating the masking layers in separate sputtering operations is the abrupt compositional interfaces between the layers. Such abrupt interfaces suffer from brittleness and poor adhesion. In addition, the layers of different composition etch at different rates during formation of the circuit pattern in the film thereby creating defects such as antireflective layer overhang, and rough line edge profile, in the etched pattern.
Efforts to eliminate the foregoing defects have focused on adding other layers to the opaque film and changing the composition of the light-blocking, chrome based layer and/or the antireflective layer. See, for example, U.S. Pat. No. 4,530,891 to Nagarekawa et al., U.S. Pat. No. 4,563,407 to Matsui et al. and U.S. Pat. No. 4,720,422 Shinkai et al.
A process approach to eliminating blank defects is disclosed by Nagarekawa et al. in U.S. Pat. No. 4,530,891. The Nagarekawa process uses the same chromium target to sputter a light-blocking chrome-carbide layer on the transparent substrate and an antireflective chrome oxide layer on the chrome-carbide layer. Deposition of the chrome-oxide layer follows in the same sputter chamber the deposition of the chrome-carbide layer. The different compositions of the layers are the result of charging the sputter chamber with different gas mixtures. First, the chamber is filled with a gas including methane and an inert gas to sputter the chrome-carbide layer. Then, the chamber is filled with a gas including oxygen and the chrome-oxide layer is deposited. Such a process would be expected to reduce contamination caused by movement of the blank substrate in the atmosphere. But, the abrupt composition interface is still a problem. And the Nagarekawa process is a batch process, requiring that the substrate sit stationary in the vacuum chamber during both sputtering steps and the change in gas mixture.